Evolution of Sunlike Stars

Our Sun formed 4.6 billion years ago from a giant cloud of interstellar gas and dust, which was located in the equatorial plane of our Galaxy. The cloud contained mainly hydrogen and helium. The temperature inside the cloud was very low and fluctuated around a few tens of degrees above the absolute zero.



Due to an external stimuli (eg. an explosion of a nearby star), the smaller part of the large gas-dust cloud began to shrink. And because this smaller part of the shrinking cloud was rotating, a flat disk called the protoplanetary disk arose from it in a few hundred thousand years. As a result of the shrinkage the cloud got also very heated, mainly in its center, where it was the thickest. There it was, where the Sun originated from the accumulated material. The rest of the protoplanetary disk then formed the planets and all the other bodies of the Solar System.



After about 50 million years in the interior of the young and hot Sun the thermonuclear reactions were ignited (burning hydrogen into helium) and the Sun became the so-called main-sequence star. At this stage it remains until today and will stay as it is for another 6.5 billion years. At the end of this stage the Sun depletes the hydrogen fuel in its core.The nuclear reactions will be therefore moved into the layer surrounding the core, which will still contain enough hydrogen. In another 7.7 billion years the Sun will turn because of the changes in its core into a so-called red giant. Its diameter will be enlarged about 166times and will extend beyond the orbit of the planet Earth. Over the next several hundred million years the Sun will throw away its outer envelopes, which will for a few tens of thousands years create a slowly-dissolving planetary nebula. At the place of the former Sun only a small, bare and extremely dense and hot core will remain ‒ the so-called white dwarf.